实体框架将外键更新为空，无查询，无相关实体的ID [英] Entity framework update foreign key to null without query and without id of the related entity

问题描述

使用EF 6.1，我想将外键设置为null（打破两个实体之间的关系），而不首先查询任一对象。最好，我也不想提供相关实体的ID（因为这需要将其添加到我的HTML中）。

Using EF 6.1, I want to set a foreign key to null (break the relation between two entities), without querying for either object first. Preferably, I also don't want to supply the id of the related entity (since that requires adding it to my HTML).

我当前的代码不做不必要查询，但需要相关实体的ID：

My current code doesn't do unnecessary queries, but requires the id of the related entity:

public void RemoveCurrentTeacherOfGroup(int groupId, int teacherId)
{
    var group = new Group { Id = groupId };
    var teacher = new Teacher { Id = teacherId };
    group.Teacher = teacher;
    _dataContext.Groups.Attach(group);
    _dataContext.Teachers.Attach(teacher);

    group.Teacher = null;

    _dataContext.SaveChanges();
}

但是，我不应该需要teacherId，groupId是足够的信息。

However, I shouldn't need the teacherId, the groupId is enough information.

更多信息：数据库是首先生成的。实体定义如下：

Further information: the database was generated code-first. The entities are defined like this:

public class Teacher
{
    public int Id { get; set; }
    ..
    public virtual List<Group> Groups { get; set; }
}
public class Group
{
    public int Id { get; set; }
    ..
    public virtual Teacher Teacher { get; set; }
}

有没有办法将外键设置为null，而没有teacherId？

Is there a way to set the foreign key to null without the teacherId?

此外，许多令人遗憾的答案使得代码不那么冗长，我觉得这些对于像外键设置为空的东西来说太多了。 / p>

Also, many kudos for an answer that makes the code less verbose, I feel these are far too many lines for something as simple as setting a foreign key to null.

推荐答案

如果您将外键添加到课程中，生活通常会更加容易：

Life is generally much easier if you add the foreign key to the class:

public class Group
{
    public int Id { get; set; }
    public int? TeacherId { get; set; }
    public virtual Teacher Teacher { get; set; }
}

然后你的方法是：

public void RemoveCurrentTeacherOfGroup(int groupId)
{
   var group = dataContext.Groups.Find(groupId);
   if(group == null)
     return;
   group.TeacherId = null;
   _dataContext.SaveChanges();
}

参考文献：

为什么实体框架将现有对象重新插入到我的数据库？

做缺少的外键

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